1. Field of the Invention
The present invention relates to a feed screw and a method for producing the same. In particular, the present invention relates to a feed screw which is used, for example, as a means for transmitting driving force for an actuator for transporting a workpiece by transmitting the driving force of a driving source to a movable member via a driving force-transmitting means to displace the movable member in a predetermined direction, and a method for producing the same.
2. Discussion of the Background
Various actuators have been hitherto used to transport the workpiece. Such an actuator has a movable member which is displaceable in the longitudinal direction of a frame for constructing an outer frame, and a feed screw such as a ball screw or a sliding screw which is coaxially coupled to a motor shaft. When the rotary driving force of a motor is transmitted via the feed screw to the movable member, the movable member is displaced linearly. The workpiece, which is placed on the movable member, is transported in accordance with the displacement of the movable member as described above.
Those ordinarily used as the feed screw are able to maintain the sliding performance in a well-suited manner, including, for example, those made of metal such as carbon steel and stainless materials, and those made of synthetic resin such as polyacetal.
However, the feed screw, which is composed of carbon steel or stainless material, involves such an inconvenience that the attachment operation is difficult, because the feed screw has a relatively heavy weight. Further, the feed screw of this type affects the magnetic field, for example, in such a way that the magnetic flux density is changed. Therefore, the feed screw of this type involves such an inconvenience that the attachment place is restricted.
On the other hand, the feed screw, which is made of synthetic resin such as polyacetal, causes the following problem. That is, any abrasion occurs at the sliding portion of the feed screw due to the use for a long period of time. As a result, the rotary driving force of the motor is not successfully transmitted to the movable member.
In order to avoid the abrasion as described above, it is preferable to apply lubricating oil such as grease to the sliding portion of the feed screw. However, the following inconvenience arises. That is, it is impossible to use any lubricating oil in an environment in which the cleanness is required so that the contamination of dust is avoided as strictly as possible, for example, in a clean room for the equipment for producing semiconductors.
The present invention has been made in order to solve the problems described above, an object of which is to provide a feed screw and a method for producing the same which make it possible to reduce the abrasion of respective members and the frictional resistance at the sliding portion wherein the weight of the feed screw is reduced to facilitate the attachment operation, and there is no limitation for the attachment place.
According to the present invention, there is provided a feed screw comprising a driving force-transmitting shaft and a bearing member externally fitted to the driving force-transmitting shaft, wherein the driving force-transmitting shaft is preferably made of aluminum or aluminum alloy, and the bearing member is preferably made of any one of synthetic resin, aluminum, or aluminum alloy.
In the present invention, preferred examples of the synthetic resin include, for example, polyacetal and super high molecular weight polyethylene.
Accordingly, the abrasion of the driving force-transmitting shaft and the bearing member for constructing the feed screw is decreased, and the sliding resistance at the sliding portions of the both is decreased. Thus, the frictional resistance is reduced. The weight of the feed screw can be also decreased, and hence it is easy to perform the attachment operation for the actuator or the like provided with the feed screw. Further, it is possible to allow the driving force-transmitting shaft to have a lengthy size. The input torque is increased owing to the large load inertia. Therefore, it is possible to realize a compact size of a motor as a driving source. Further, the influence exerted on the magnetic field is suppressed, and hence the attachment place is not limited.
In the feed screw according to the present invention, it is preferable that a nickel coating film is formed on a surface of the member made of aluminum or aluminum alloy by means of a plating treatment including an electroless nickel plating treatment. It is preferable that a thermal hardening treatment is further applied to the layer having been subjected to the plating treatment. Alternatively, it is also preferable that the surface of the member made of aluminum or aluminum alloy is surface-treated by means of an aluminum anodic oxidation treatment or a magnesium anodic oxidation treatment.
The hardness and the smoothness of the surface of the member made of aluminum or aluminum alloy are enhanced by means of the treatment as described above. It is possible to further reduce the abrasion of the respective members and the frictional resistance of the sliding portions. Especially, when the member, which is surface-treated by means of the anodic oxidation treatment, is subjected to the thermal hardening treatment, it is possible to obtain such an effect that the stress is relaxed for the anodic oxidation-treated layer and the base metal layer made of aluminum or aluminum alloy. The anodic oxidation treatment may be a colored anodic oxidation treatment. Further, a sealing treatment, which is performed in a slight degree as compared with ordinary cases, may be applied to the layer having been subjected to the anodic oxidation.
It is also preferable that a surface of the member made of aluminum or aluminum alloy is surface-treated by means of a shot peening treatment applied a plurality of times by alternately using two types of particles having different particle sizes. The shot peening treatment may be also applied to the member which has been surface-treated by means of the plating treatment or the anodic oxidation treatment. The shot peening treatment makes it possible to obtain an excellent feed screw in which the surface roughness and the residual stress are reduced.
When the shot peening treatment is applied a plurality of times by alternately using the two types of particles having the different particle sizes, the particles having the large particle size are firstly used to harden the surface. Subsequently, the particles having the small particle size are used to form fine irregularities on the surface. The irregularities function as oil reservoirs as described later on.
In this case, it is preferable to apply, between the respective shot peening treatments, a surface finish treatment such as a liquid honing finish treatment, a lapping finish treatment, and a sandblast finish treatment.
The following method is preferably used as a method for reducing the surface roughness and the residual stress by means of the plurality of times of the shot peening treatments. That is, the size of the shot peening particle, which resides in the relatively large particle and which is optimum for the aluminum or the aluminum alloy as the material to be processed, is experimentally determined and programmed beforehand. The shot peening treatment is performed under the obtained condition. Subsequently, the shot peening treatment is performed by changing the shot peening condition including the particle size, in a stepwise manner ranging from the surface to the inside of a workpiece. It is desirable that these processing operations are automated.
The stepwise shot peening treatment as described above may be performed once, twice, or more, for example, before the processing treatment for plating the feed screw. When the sandblast finish treatment, the buff finish treatment, or the liquid honing finish treatment is performed before, during, or after the stepwise shot peening treatment, it is possible to remove fine burrs on the workpiece surface, which is preferred.
In the method for effecting the surface treatment or the surface finish treatment by means of the shot peening treatment, it is preferable to form the inclined surface or the oil reservoir having the elastohydrodynamic lubrication effect on the surface of the feed screw. Therefore, when the shot peening treatment is performed, it is preferable to use particles having a ball-shaped configuration or a polyhedral configuration with a size which makes it possible to form the inclined surface or the oil reservoir having a desired size.
When the portion, which is located in the vicinity of the surface of the member, is recrystallized or hardened by means of the shot peening treatment, or when the portion is thermally hardened by means of the partial heat generation, then it is possible to allow the surface of the member to have a higher hardness. When the shot peening treatment is performed, the metal, which is contained in the particles used for the shot peening, penetrates into the surface of the member. Therefore, it is also possible to obtain an effect to improve the quality of the surface owing to the permeation of the metal.
It is preferable that the member, which is surface-treated by means of the plating treatment, the aluminum anodic oxidation treatment, the magnesium anodic oxidation treatment, or the shot peening treatment, is subjected to a surface finish treatment by means of at least one method selected from the group consisting of a super finishing treatment, a buff finish treatment, a lapping finish treatment, a liquid honing finish treatment, a burnishing finish treatment, a roller finish treatment, a chemical polishing finish treatment, an electropolishing finish treatment, a sandblast finish treatment, a grinding finish treatment with a diamond grinding wheel or a boron nitride grinding wheel, a grinding finish treatment with a diamond grinding wheel or a boron nitride grinding wheel equipped with a dressing mechanism, and a cutting finish treatment with a diamond electrodeposition tap or a broach tap. Accordingly, for example, the smoothness of the surface of the member can be further improved.
According to the present invention, there is provided a to method for producing a feed screw, wherein at least any one of a driving force-transmitting shaft and a bearing member is preferably manufactured from aluminum or aluminum alloy by means of a rolling method. That is, a cold process or a hot process is performed while rotating the aluminum or the aluminum alloy, and pressing a die against the aluminum or the aluminum alloy to form at least any one of the driving force-transmitting shaft and the bearing member.
Accordingly, it is possible to efficiently produce the feed screw having a high machining accuracy.
According to the present invention, there is also provided a method for producing a feed screw, wherein at least any one of a driving force-transmitting shaft and a bearing member is preferably manufactured from aluminum or aluminum alloy by means of a precision cutting method based on the use of any one selected from the group consisting of a diamond tip, a boron nitride tip, an electrodeposition tool made of diamond, a sintering tool made of diamond, an electrodeposition tool made of boron nitride, and a sintering tool made of boron nitride.
Accordingly, it is possible to further reduce the abrasion between the driving force-transmitting shaft and the bearing member and the sliding resistance at the sliding portions of the both.